Various methods for extracting and recovering metals, particularly precious metals, present in complex source materials such as ores and the like are known in the art.
In the case of gold, for example, one well-established method for extracting the metal from its ore involves crushing the ore, treating the gold containing pulp with a solution of a cyanide and removing the gold in the form of a complex with cyanide ions. The gold is subsequently recovered from the complex by a precipitation reaction, for example with metallic zinc or by electrolysis.
Although cyanide leaching processes of this type have found widespread application in the past, they are potentially hazardous to operate and present serious environmental concerns. Indeed, such is the current level of concern that such process are no longer operated in many territories. Moreover, these methods are not generally applicable to other precious metals; for example, silver and copper cannot effectively be extracted using cyanidation technology.
Concerns about the environmental impact of the processes involving the use of cyanides have led to considerable interest in the development of alternative, commercially more attractive and environmentally less damaging methods for extracting metals, particularly precious metals, from the materials in which they are contained.
This has led to a revival of interest in chlorination-based extraction processes whereby the metal is extracted from the material in which it is deposited by the formation of volatile chloride containing metal compounds from which the metal can be recovered. Chlorination-based processes are well known in the art and methods which have been described in the literature include the leaching of metal ores using aqueous solutions of chlorine, chlorides and hypochlorites as discussed, for example, in U.S. Pat. No. 4,353,740 and U.S. Pat. No. 5,169,503.
Other methods commonly used for extracting precious metals from ores include autoclaving. Although this method is currently commercially used in the mining industries, the associated capital costs represent a considerable disadvantage of the method.
There therefore remains a continuing need for the development of further, improved, commercially viable methods for extracting metals, especially precious metals, from ores, concentrates or other materials in which they are found. In naturally occurring deposits of precious metals, the metal is commonly present at too low a concentration to be extracted profitably using existing methods. Methods for extraction of precious metals which avoid the environmental hazards associated with conventional cyanide leaching and which may be used in situations where cyanide leaching is ineffective are particularly sought.